DESIGNING THE WIRELESS LAN LAYOUT
The organization must consider several issues about the layout of the stations, -
including the mode (topology) of the WLAN, the coverage area, the number of users, network utilization, the number and type of physical obstructions and noise levels.   Peer-to-Peer Versus Infrastructure Mode
Radio-based WLANs can be set up in one of the two modes: peer-to-peer or infrastructure. In peer-to-peer mode, the wireless stations communicate directly among themselves without using an access point. Peer-to-peer mode is also known as ad hoc rode or Independent Basic Service Set (IBSS). In the infrastructure mode, wireless Stations communicate through one or more access points. A network using just one access point is known as Basic Service Set (BSS): two or more BSS wireless networks with multiple access points can be combined to form an Extended Service Set (ESS).
The decision regarding whether to configure WLAN for peer-to-peer or infrastructure mode should be based upon the purpose of the network. Peer-to-peer mode (IBSS) should be used when wireless stations need to communicate only with each other. This mode is good for a wireless network that must be installed quickly where a network does not already exist, such as in hotel room or convention centre. However, IBSS wireless stations can communicate only among themselves, and there is no access to larger network or to the Internet.
A Basic Service Set (BSS) WLAN should be used when one access point can cover the service area for the stations and users cannot roam beyond this area. However, if the service area is too large for one access point or users must roam outside of this area while still maintaining a network connection, then multiple access points should be used. Such a station, thus, calls for an Extended Service Set (ESS) WLAN.

Coverage Area
Determining the size of the coverage area is an important first step in designing the layout of WLAN. The maximum distance that stations can be apart varies depending upon the setting. Table 9.2 summarizes these distance for DSSS.

The differences are due to the wide range of factors that affect wireless transmissions. In a typical semi-open indoor environment, the maximum distance is about 375 ft (115 m), whereas in a truly open environment it can be as far as 1000 ft (304 meters). This is true for both WLANs that use an access point (infrastructure mode)-as shown in Figure and those that do not (peer-to-peer mode)-as shown in Figure .

When designing the layout for a station, it is very important to note that the distance between the stations directly affects the speed of the network. Table 9.4 illustrates typical speeds in the semi-open environment, which is defined as workspace divided by shoulder-height, hollow walls.

A station with an 11 Mbps wireless NIC can communicate with other clients up to a distance of 375 feet (115 metres) in a semi-open environment. However, only stations within the first 165 feet (50 meters) can communicate at 11 Mbps. Stations that are between 300 to 375 feet (90 to 115 metres) away will communicate at only 1 Mbps as illustrated in Figure for a WLAN that uses an access point (infrastructure mode) and in Figure for a network that does not (peer-to-peer mode).

This ability to adjust the data rates automatically to compensate for the changing nature of the radio signals is known as dynamic rate shifting. Users close to the access point or another station would connect at the full 11 Mbps rate. However, if a station roams beyond the optimal range for 11 Mbps operation or if there is interference from another source, the station will transmit at lower speeds (5.5, 2 or 1 Mbps). When the station moves closer, the connection will automatically speed up again.

BACK